The present invention relates to an illuminating device capable of illuminating an object uniformly and with improved efficiency and applicable to an electrophotographic copier, electrostatic printer or similar image recording equipment of the type illuminating the entire surface of an object at a time.
A prerequisite with image recording equipment of the type described is that a photoconductive element be exposed to a document image by uniform illumination over the entire surface thereof. To meet this requirement, some different approaches have been proposed such as increasing the illumination around an area to be illuminated in consideration of the cosine fourth-power rule of a lens, and disposing a correcting filter or similar element on an optical path while uniformizing imagewise light incident to a lens. With the second-mentioned approach, it is necessary to uniformly illuminate an expected area when the diffuse reflection of the surface of a document is uniform. An implementation for such uniform illumination is disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 61-93501. The device disclosed in this Laid-Open Publication is made up of a light source, and a main mirror having a reflecting surface implemented as a surface of revolution. The axis of revolution of the reflecting surface is positioned outside of one end of an area to be illuminated and is coaxial with the center of the light source and perpendicular to a flat portion of the expected area. The configuration of the reflecting surface is such that light issuing from the light source is reflected toward the flat portion with a luminous flux density which is proportional to the distance as measured from the axis of revolution. Although this kind of illuminating device is successful in illuminating a flat area uniformly, it cannot achieve a satisfactory illuminating effect because the reflection from the main mirror is emanated radially. More specifically, not the whole light is used to illuminate the desired area, i.e., a part of the light is emanated to the outside of the desired area.
Apart from the specific construction described above, when a main mirror is located at one end of an area to be illuminated which is rectangular and flat, the illumination efficiency is lowered substantially by the light which misses the expected area due to the emanation of the reflection.